The invention relates to a device composed of a light guide plate, in which light emitted from a light source is guided by components in a direction parallel to the main surfaces of the plate and issues from projections on a surface of the plate.
With a known device of this type (German Offenlegungsschrift 3,240,942), the projections are situated spaced out directly on the plate. No details are given on their size and spacing. For the exemplary embodiments i is assumed that these values are in the centimeter range. The reflection of the light from the surface of the plate is uneven because only the regions of the surface which have projections emit light.
The underlying object of the invention is to develop the known device further in such a way that the resulting density of surface illumination is even to the eye.
According to the invention this object is achieved by the fact that the smooth surface of the plate is coated with a continuous, transparent intermediate layer having on one side closely spaced projections with a height and width lying in a range between 0.5 mm and 0.001 mm.
Since, according to the invention, the sizes and spacing of the light regions in the form of the projections are of such a nature that their upper limits coincide approximately with the lower limit of the resolving power of the human eye (this resolving power is present when the distance between two points is greater than 0.2 mm when the points are located at a distance of 1 m from the eye, see Grimsehl, Lehrbuch der Physik, vol. 3, 1955, page 309), the main surface of the device according to the invention appears to be evenly luminous. The provision of the intermediate layer on the other hand enables the projections to be produced; without the intermediate layer the projections of the type according to the invention could not be created. The lower limit for the sizes of the projections is thus meaningful since smaller projections could hardly, if at all, be manufactured.